1. Field of the Invention
This invention relates generally to wire bonding, and specifically to a method and apparatus for the production and quality control testing of an ultrasonically bonded wire connection.
2. Description of the Related Art
Bonding is an extremely widely used method of connecting wires in electronic devices of all kinds, in order to form contacts with electronic components and in particular integrated circuits (chips). The quality of these connections determines to a considerable extent the performance and reliability of the relevant electronic devices. Therefore the manufacturers of these devices pay great attention to monitoring the quality of the connections, and the manufacturers of wire-bonding machines are faced with a demand for ever more reliable testing and process-control systems.
One of the furthest developed and most widespread bonding methods is ultrasonic (“wedge”) bonding, which basically represents a micro-version of friction welding. Here—as described, for example, in the applicant's patent U.S. Pat. No. 4,619,397—an aluminium wire in contact with a substrate surface, to which it is to be connected in such a way that the two materials remain in contact, is put into rapid oscillation by an ultrasound transducer and simultaneously pressed against the surface. Under the influence of the compression (bonding) force and the oscillation (bonding) power, an oxide coating on the surface is broken up and, as severe deformation and local heating occur, the wire and surface materials intermingle to produce bonding at the interface between wire and surface.
A more detailed description of this method is unnecessary here, as it has long been well known to those skilled in the art.
For testing bonded connections produced in this way, many methods have been established, of which reference is made here only to that disclosed in the applicant's patent U.S. Pat. No. 4,984,730. This document describes a test method based on monitoring a deformation of the wire during the bonding process and comparison with a standard or reference curve. When the measured deformation curve is too far from the reference curve, this is regarded as indicating inadequate quality of the bonded connection, and when such unacceptable deviations are detected, the procedure should be interrupted and the relevant process parameters readjusted.
A method of testing connections produced by ultrasonic bonding that is similar, in a certain sense, is described in the patent DE 44 47 073 C1. Here the firmness of the connection is the decisive parameter for the bond quality. It is proposed that the relevant parameter to be monitored is the velocity or time course of deformation of the wire to be bonded and the time course of the amplitude of the bonding tool (wedge amplitude) during the bonding process; the results of this monitoring are to be compared with reference data. This method enables the firmness of each individual connection to be tested with no substantial extra expenditure of time.
In the applicant's patent U.S. Pat. No. 5,314,105 a system is proposed for controlling an ultrasonic wire-banding process in-real time, or quasi-real time, by monitoring the time dependence of deformation of the bonded wire. Specifically, it is proposed that the power input to the ultrasound transducer should be kept at a high level until a marked increase in deformation appears in the time-dependence curve, at which point the power is reduced to a predetermined lower level. Furthermore—in a preferred procedure—it is proposed to turn the ultrasound transducer off completely when the wire deformation has substantially reached a predetermined final value.
In the applicant's patent GB 2 271 305 B a procedure for controlling a soldering or bonding process of the reflow type is proposed, in which the power applied to drive prespecified sections of the process is adjusted to different levels. In this document the application of this principle for a wire bonder of the kind cited above is also mentioned.
The previously proposed methods and apparatus have proved not to provide optimal results in all of the constellations of materials and processes encountered in practice. It has been impossible to control or regulate the bonding process in the initial phase (the first five to ten ms), because during this period the wire to be bonded was not deformed sufficiently to permit evaluation.